Add a consolidated file cache

This commits reworks how file caching is performed in Hugo. Now there is only one way, and it can be configured.

This is the default configuration:

```toml
[caches]
[caches.getjson]
dir = ":cacheDir"
maxAge = -1
[caches.getcsv]
dir = ":cacheDir"
maxAge = -1
[caches.images]
dir = ":resourceDir/_gen"
maxAge = -1
[caches.assets]
dir = ":resourceDir/_gen"
maxAge = -1
```

You can override any of these cache setting in your own `config.toml`.

The placeholders explained:

`:cacheDir`: This is the value of the `cacheDir` config option if set (can also be set via OS env variable `HUGO_CACHEDIR`). It will fall back to `/opt/build/cache/hugo_cache/` on Netlify, or a `hugo_cache` directory below the OS temp dir for the others.
`:resourceDir`: This is the value of the `resourceDir` config option.

`maxAge` is the time in seconds before a cache entry will be evicted, -1 means forever and 0 effectively turns that particular cache off.

This means that if you run your builds on Netlify, all caches configured with `:cacheDir` will be saved and restored on the next build. For other CI vendors, please read their documentation. For an CircleCI example, see 6c3960a8f4/.circleci/config.yml

Fixes #5404

hugolib/filesystems/basefs.go

@@ -81,7 +81,6 @@ type SourceFilesystems struct {
 	Layouts    *SourceFilesystem
 	Archetypes *SourceFilesystem
 	Assets     *SourceFilesystem
-	Resources  *SourceFilesystem
 
 	// This is a unified read-only view of the project's and themes' workdir.
 	Work *SourceFilesystem
@@ -376,13 +375,6 @@ func (b *sourceFilesystemsBuilder) Build() (*SourceFilesystems, error) {
 	}
 	b.result.Assets = sfs
 
-	sfs, err = b.createFs(true, false, "resourceDir", "resources")
-	if err != nil {
-		return nil, err
-	}
-
-	b.result.Resources = sfs
-
 	sfs, err = b.createFs(false, true, "", "")
 	if err != nil {
 		return nil, err